From a purely Christian perspective, a sect would be a group of Christians that are not entirely mainstream or fundamental in their beliefs, but are not so far removed from the orthodoxy to be heretical. Seventh Day Adventist would be an example of a sect.
The term cult is universally used as a negative term. Christians would define a cult as any group of people who's beliefs would be classified as heretical to traditional orthodoxty. For example, Mormons and Jehovah's Witnesses would be cults. They deny the Trinity.

The word "sect" is generally used to refer to the various divisions of religious denominations of a particular faith without any special distinction as to its special doctrines or beliefs.It may also be a term by which a splinter group may be designated in relation to its original or main group from which it broke away. Most major religions have their sects that vary in identity according to their interpretation of the scriptures or belief system.
For example the Suni and Shia are traditional Islamic sects in the same way that a Baptist is to a Presbyterian but ISIS and Al Qaeda are ultra radical sects of Islam who are not recognized by conservative Islam because of their revolutionary and violent means of advancing Islam.
Within the Christian world, a sect is a group that simply holds onto some particular set of doctrines or religious practices as being more fundamental to their faith. These practices are usually biblical.They simply over-emphasize on the significance of these teachings above others. They however do not deny the truths of Scripture generally or reject the "mainstream" churches as false.
Cults on the other hand are usually splinter or independent religious groups that deny the fundamental biblical truths held by mainstream churches. Many Christian cults have a problem with the Trinitarian revelation of God in Three Divine Persons and several other fundamental truths revealed in the Bible. Some deny the divinity of Jesus Christ and/or the Holy Spirit while others elevate some Bible characters and New Testament church saints to divine status in total violation of the truths taught in the Scriptures. Still others have added their own literature or church traditions to the teachings of Scripture which they considered to be of the same standing or even superior to the Bible itself.
Some cults have also abandoned or relegated the authority of the Bible and instead uphold the teachings of their cult leaders as fundamental/additional source of revelation from God. They will usually hold their "inspired" religious literature side by side with the Bible as the source of reference on matters of their faith. Their cult leader, whether living or dead, is highly revered and his/her teachings are law to the faithful even where they are unbiblical.
Some cults mistakenly belief that they are the only group through which men may discover or reach God. This exclusivism is a trait identified with a section of the SDA faith who argue that keeping the Sabbath is a fundamental practice of the Christian faith and that they are the sole "remnant" who obey God in this context as required in the Bible. This claim is unbiblical in every essence because whereas worshiping on a Sabbath is biblical, the Bible does not define a specific meeting day for Christian believers to worship God.
Some sects and splinter groups can mutate into cults or neo-cultic movements over time depending on the type of leaders who rule them. The neo-cultic movements may have been genuine sects who unfortunately assimilated some cultic teachings along the way and mixed them with the biblical truths that they originally held. The dangerous concoction that emerges from the blend is a foul stench that further isolates them from the way of the Cross.
While neo-cultic teachers will not openly deny fundamental scripture teachings, they however add some false teachings to "supplement and expand" these truths such that they ultimately result in serious distortions of scriptural truths. For example one will hear a neo-cultic teacher lift a verse out of its Bible context are make a false interpretation of a Bible passage and simply misapply it to suit his preconceived idea. Unfortunately such teachers will vehemently reject biblical correction for their distortion even where the correction is legitimate. Besides most neo-cultic teachers reject biblical training and this largely contributes to their poor handling of Scripture. Surprisingly some